1. Field of the Invention
The present invention relates to an applied-voltage fuzzy control process for induction motors and to a related fuzzy controller.
2. Discussion of the Related Art
As is known, an electric drive system is a system capable of converting electric power supplied in input into mechanical power. These systems generally include three main subsystems: an electric motor, a control system and a power system. The electric motor is the element which enable conversion of the energy. The development of drive circuits that has taken place in recent decades has increased interest at the industrial level in the class of alternating-current motors. The characteristics that cause them to be preferred are mechanical strength, low moment of inertia, and low manufacturing and maintenance cost.
The control system must generate the control signals on the basis of a comparison between measured signals and reference signals, simultaneously compensating for parametric changes produced for example by saturations and/or temperature variations.
Control systems can be produced using digital and analog technology. The current trend is to develop digital control systems, which achieve better performance, have sophisticated control techniques, and flexibility in interfacing with automation and diagnostics systems.
The power block, which supplies power to the motor in the manner described by the control system, includes an inverter used in AC drives. Inverters can be classified according to the variable that they control, and thus as applied-current and as applied-voltage inverters, or according to the waveform of the applied variable.
FIG. 7 illustrates a known control system. This figure shows the various blocks that compose the control system. The blocks include the conversion block, the control block, the signal-command separation block, and the power block.
The conversion block includes a transducer 1 (encoder) which is suitable to convert the speed of an AC electric motor 2 into an electric signal. These signals, which are generally analog, are transferred to the processing units.
The control block, including the microcontroller 3, receives the electric signals, appropriately converted into digital form by an A/D converter 8, from the conversion block, and determines, on the basis of these signals, the sequence of the control signals to be transmitted to the gates of the power devices 4 of the power block so as to apply the desired power supply frequency and voltage to the AC motor 2.
The signal-command separation block, including two interfaces 5 and 6 and pulse transformers or optoisolators 9 interposed between them, prevents the switchings that affect the power switching devices from introducing noise in the signal section, i.e. in the control logic section.
The power block, which includes the power device 4 (triac, IJBT, MOSFET, etc.), applies the desired set of three voltages to the phases 10 of the motor.
The speed control of the AC motor 2 within the allowable value range can be divided into two sections: the first section provides a constant-torque operation, whereas the second section provides a constant-power operation. A frequency-controlled asynchronous motor operates with constant torque if a proportionality rule is set between the voltage V and the power supply frequency w; this rule is expressed by the function V=Kw, where K is a constant.
The advantage linked to the use of this control technique is simplicity in execution, which reduces operating costs, especially when used in industrial-type systems. However, these systems are not very precise and resistant, and their use is therefore limited.
In order to obviate these problems, solutions have been proposed where additional measurements are performed, introducing compensation devices that correct parameter changes and solve stability problems. The use of this type of strategy compromises the essential characteristics of this type of control, i.e. simplicity and cost.
Known AC motor control systems have other drawbacks.
For example, locking onto a given set point is often too slow, especially when it is necessary to have small overshooting. Also, the locking time is too long in present systems when the operating conditions vary and it is desirable to have a small steady-state error.
One aim of the present invention is to provide an applied-voltage fuzzy control process for induction motors and a related fuzzy controller which can improve the performance of conventional control systems.
Within the scope of this aim, an object of the present invention is to provide a process that allows fast locking onto a given set point with small overshooting.
Another object of the present invention is to provide a process which allows a minimal locking time when the operating conditions vary.
Another object of the present invention is to provide a process which allows a limited steady-state error.
Another object of the present invention is to provide a process which provides more robust control with respect to known systems.
Another object of the present invention is to provide a process, and the device for performing it, which are highly reliable and are relatively easy and inexpensive to manufacture.